Sphingolipidoses are a group of congenital disorders caused by the impaired catabolism of glycosphingolipids. Since the storage of partially degraded glycosphingolipids in the secondary lysosomes of various sphingolipidoses is linked to a deficiency of glycosidases, the enzyme deficiency was once considered to be the sole cause of the pathogenesis of sphingolipidoses. Through the past support of this grant, we have established that activator proteins are requisite for the enzymic hydrolysis of GM1 and GM2. We have also discovered a new variant of Type-AB GM2 gangliosidosis caused by a defect in Beta-hexosaminidase A. Our work, together with that of others, has resulted in the introduction of a new concept that both glycosidases and activator proteins are required for the catabolism of glycosphingolipids. Therefore, according to this new concept, sphingolipidoses can be caused by four different etiologies: a defect in either activator protein or glycosidases or a deficiency in either activator protein or glycosidase. In this grant period we would like to continue investigating the chemical pathology of sphingolipidoses by studying the catabolism of glycosphingolipids. Our specific aims are: a) to improve the isolation techniques for activator proteins which stimulate the hydrolysis of GM1 and Gm2 (GM1- and GM2-activators) and to further study their specificities and subcellular localization; b) to characterize activator proteins in human urine; c) to develop a sensitive method such as enzyme-linked adsorbent assay or radioimmunoassay for the detection of GM1- and GM2- activators and the clinical diagnosis of lipidosis due to the deficiency of activator protein; d) to complete the overall catabolic pathways of GM1 and GbOse4Cer; e) to study inhibitor proteins which inhibit the catabolism of glycosphingolipids, and f) to continue the isolation and characterization of glycosidases pertinent to sphingolipidoses. Our long term objective is to completely understand the role of activator proteins, inhibitor proteins, and their interaction with glycosidases and glycolipid substrates. This information will lead to the better understanding of the catabolism of glycosphingolipids and the molecular basis of sphingolipidoses.